Galactic archaeologists find stars in the Milky Way came from another galaxy 12 billion years ago

An illustration shows a stage in the predicted merger between our Milky Way galaxy and the neighboring Andromeda galaxy, as it will unfold over the next several billion years. NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

Look up on a dark night and the Milky Way appears as a calm band of stars across the sky. But our galaxy has a violent past.

For more than 13 billion years it has grown through star formation and mergers with other galaxies. We already know that several substantial galaxies helped build the young Milky Way.